We use 1 pump with 2 reels, the pump goes before the heater, the flow rate does go up on one reel, when the other is turned off, but it is not instant, take 10-20 seconds to see the increased flow.
There way a guy on here that ran 2 pumps from one heater, but he had the pumps after the heater pulling the water through, I'm not sure how well it worked, he wasn't the type of guy that would have anything that didn't work properly so it was probably ok.